Dune Slacks — The Wetland in the Middle of the Dunes

19 March 2026

What a dune slack is

A dune slack forms when wind erosion lowers the sand surface down to the level of the water table. The result is a shallow, flat-bottomed depression that's either permanently damp, seasonally flooded, or occasionally underwater. The water table on the Sefton Coast sits relatively close to the surface in many areas, so slacks can form anywhere erosion is active.

Slacks vary in size from a few square metres to several hectares. They range from almost permanently dry (dry slacks, with moisture-dependent grassland) through to seasonally wet and permanently waterlogged. Each condition supports different communities of plants and animals.

The dune system at Ainsdale and Birkdale holds one of the best examples of a dune slack series in England — multiple slacks at different successional stages, from newly formed open water through to mature willow scrub enclosing the water. This range of stages within a small area is what makes the site nationally important.

The plants

Dune slack vegetation is distinct from the surrounding dune system. In wet slacks, look for Creeping Willow (low-growing, spreading along the ground rather than growing into a tree), Marsh Pennywort (round, lily-pad like leaves on long stems across wet ground), and in the wetter areas, Common Reed and Bulrush.

The drier slack margins are where the orchids appear. Northern Marsh Orchid and Bee Orchid both occur in the Ainsdale NNR slacks. June is the month for orchids on the Sefton Coast — the displays in a good year are significant, particularly in the dry slack grassland areas.

Round-leaved Wintergreen is an indicator of old, stable dune slack — a small, low-growing plant that only occurs in habitats with a long, undisturbed history. Where you find it, you're standing in habitat that's been ecologically intact for a long time.

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A dune slack in the Ainsdale NNR complex — standing water between dune ridges supporting aquatic vegetation. The water table here is at or near the surface year-round.

The animals

Dune slacks are the breeding habitat for Natterjack Toads on the Sefton Coast. The females lay eggs in shallow, warm, permanent or semi-permanent water. The key requirement is that the water stays warm and doesn't flood quickly — the tadpoles need enough time to develop and metamorphose before any flash flooding, and the shallowness means the water heats rapidly in spring sun.

The invertebrate fauna of dune slacks is specialised and significant. Several rare hoverflies, beetles and water beetles are associated with dune slack habitat nationally, with several species having their strongholds on the Sefton Coast.

Waders and waterfowl use the larger, wetter slacks as feeding and roosting habitat. Snipe probe for invertebrates in the soft slack substrate in winter. Teal use the more open water areas. Sedge Warbler and Reed Bunting breed in the tall vegetation margins.

Threats and conservation

The primary threat to dune slacks on the Sefton Coast is vegetation succession. Without active management, the slacks progressively fill with willow scrub — Creeping Willow transitions to Grey Willow, then to dense thickets that shade out the water and the specialist slack vegetation. Within 30–50 years, an unmanaged wet slack can become closed woodland.

Management interventions include scrub clearance, controlled grazing (cattle and Konik ponies have been used on the Sefton Coast to graze dune vegetation), and in some cases mechanical scrape of accumulated organic material. The goal is to maintain open water or damp grassland conditions rather than allowing succession to scrub and woodland.

The dune system is also under pressure from declining water tables — reduced rainfall and groundwater extraction have lowered the water table in some areas, leaving slacks that were formerly wet now dry for extended periods. This reduces the habitat available to water-dependent species.

Where to see dune slacks

Ainsdale NNR (Shore Road, Ainsdale PR8 2QB): the core site for dune slack wildlife on the Sefton Coast. The network of paths through the reserve passes several slack complexes. Spring and early summer are the best times — orchids in June, Natterjack Toads calling from April.

Birkdale Hills Local Nature Reserve (Birkdale, Southport): accessible via the public footpath network. Less managed than Ainsdale NNR but holds good slack habitat.

What to bring: binoculars for the birds; a hand lens if you want to look at plants and invertebrates properly. Waterproof footwear — even in dry conditions the slack margins are soft and damp.